I've been asked to admin a hylafax server. Incoming faxes are emailed to
different users based on termination phone number.

Looking thru the docs it appears this config info is usually found in
FaxDispatch. I did find a FaxDispatch file in /var/spool/hylafax/etc and
it looks like this file was once used to configure this feature. Those
lines appear to be commented out now (line starts with #). Users are
receiving faxes so the feature is configured properly "somewhere".

Is there another file (or another way) that is used to configure this
information?

Undoubtedly you have a customized faxrcvd script in-use.

Look at your modem config files (/var/spool/hylafax/etc/config.<devid>)
and look for "FaxRcvdCmd". This will pinpoint the faxrcvd script that's
being used to deliver faxes. If you do not have a FaxRcvdCmd modem
config option then the previous administrator simply customized the
distributed /var/spool/hylafax/bin/faxrcvd script. (If this is the case
take care when upgrading because it will overwrite your customized
version as faxrcvd is considered part of the package distribution.)